1. Field of the Invention
The present invention relates to a wavelength demultiplexer used for optical communications.
2. Description of Related Art
In recent years, there have been considerable advances in the research and development of wavelength demultiplexers for realizing FTTH (fiber to the home) systems.
As one such wavelength demultiplexer, which is shown in FIG. 6, a reflection-type wavelength demultiplexer provided with a dielectric multilayer filter has been proposed (see, for example, 1995 Denshi Jouhou Tsuushin Gakkai Electronics Society Taikai C-229, or Shingaku Gihou EMD 96-36, CPM 96-59, OPE 96-58, LQE 96-60 (1996-08)).
FIG. 6 is a schematic top view showing waveguide portions and a dielectric multilayer filter 16 of a reflection-type wavelength demultiplexer provided with a Y-branching waveguide 14 (referred to as xe2x80x9cwavelength demultiplexer Axe2x80x9d in the following), disclosed in these documents.
When wavelength multiplexing light SP including first wavelength light S1 and second wavelength light S2 is input into an optical waveguide 18 for wavelength multiplexing light, the dielectric multilayer filter 16 transmits the first wavelength light S1 and inputs it into the Y-branching waveguide 14, and reflects the second wavelength light S2, which is input into a reflection-light waveguide 20, thus demultiplexing the wavelength multiplexing light SP.
The Y-branching waveguide 14 includes a main waveguide 22, a tapered waveguide 24 for widening the waveguide width, and first and second branching waveguides 26 and 28. After the first wavelength light S1 that has been input into the Y-branching waveguide 14 has been propagated through the main waveguide 22 and the tapered waveguide 24 for widening the waveguide width, it branches into the first and the second branching waveguides 26 and 28, and is output to the outside.
However, for the configuration of optical communication modules using such reflection-type wavelength demultiplexers, the waveguides are formed so that the input direction and the output direction of the wavelength demultiplexer coincide with one another (that is, they are parallel to each other). As will be explained in the following, the input direction and the output direction correspond to a first propagation direction L1. This means, that the main waveguide 22 is connected to the dielectric multilayer filter 16 in a second propagation direction L2, in order to reduce the emission loss of input first wavelength light S1. The center line of the main waveguide 22 bends smoothly until it runs in the first propagation direction L1. First and second branching waveguides 26 and 28, whose center lines are arranged symmetrically to one another, are connected to this main waveguide 22. At a third port P3 and a fourth port P4, the center lines of the first and second branching waveguides 26 and 28, too, run in the first propagation direction L1.
However, the wavelength demultiplexer A disclosed in the above-noted documents has the disadvantage that it has a structure that is long with respect to the first propagation direction L1.
To remove this disadvantage, a reflection-type wavelength demultiplexer has been proposed in which the dielectric multilayer filter 16 is arranged obliquely against the input direction LA (referred to as xe2x80x9cwavelength demultiplexer Bxe2x80x9d in the following), as shown in FIG. 7. Like FIG. 6, FIG. 7 is a top view showing the wavelength demultiplexer B.
However, the production efficiency for the wavelength demultiplexer B is poor. The following explains the reasons for this. FIG. 8 is a top view of a series of three chips arranged next to each other under the same orientation on a wafer for forming wavelength demultiplexers B. Grooves 36 for inserting a dielectric multilayer film (in the drawings, these grooves are indicated by hatching) are formed on the surface of the chips, but these grooves 36 are arranged at an angle, so that they are not on a common straight line and the grooves 36 on the chips of one series of reflection-type wavelength demultiplexers cannot be formed by dicing in one step. Consequently, as mentioned above, the production efficiency for the wavelength demultiplexer B is poor.
On the other hand, the production efficiency for the wavelength demultiplexer A is higher than that for the wavelength demultiplexer B. The reason for this is explained in the following. Like FIG. 8, FIG. 9 is a top view, of a series of three chips arranged next to each other under the same orientation on a wafer for forming wavelength demultiplexer A. Grooves 36 for inserting a dielectric multilayer film (in the drawings, these grooves are indicated by hatching) are formed on the surface of the chips, but these grooves 36 are formed on a common straight line, so that the grooves 36 on the chips of one series of reflection-type wavelength demultiplexers can be formed by dicing in one step. Consequently, as mentioned above, the production efficiency for the wavelength demultiplexer A is high.
It is therefore an object of the present invention to provide a wavelength demultiplexer whose overall length in the first propagation direction is short, whose production efficiency is high, and whose emission loss is smaller than that of the above-described conventional configurations.
In order to attain this object, a wavelength demultiplexer of the present invention includes a wavelength demultiplexing portion and a Y-branching waveguide. The wavelength demultiplexing portion demultiplexes a specific wavelength of light from wavelength multiplexing light that has been input into the wavelength demultiplexer in a first propagation direction and then outputs the specific wavelength of light in a second propagation direction that is different from the first propagation direction. The Y-branching waveguide outputs the specific wavelength of light, which has been input into the Y-branching waveguide in the second propagation direction, in the first propagation direction. The Y-branching waveguide includes a main waveguide, a tapered waveguide, a first branching waveguide, and a second branching waveguide. The main waveguide is connected to the wavelength demultiplexing portion. The tapered waveguide is connected to the main waveguide and widens the waveguide width. The first branching waveguide and the second branching waveguide are both connected to the tapered waveguide.
In this configuration, the main waveguide is a straight waveguide whose center line is oriented in the second propagation direction. After bending the center line of the first branching waveguide along a smooth first curved line away from the second branching waveguide, a tangential direction of the first curved line coincides with the first propagation direction. After bending the center line of the second branching waveguide along a smooth second curved line away from the first branching waveguide, and after bending it along a smooth third curved line, which is connected to the second curved line in the tangential direction of the second curved line, into a direction towards the first branching waveguide, the tangential direction of the third curved line coincides with the first propagation direction.
Moreover, when a shape of the Y-branching waveguide is seen as a xe2x80x9cYxe2x80x9d, at least one of a first condition and a second condition is satisfied.
The first condition is that an entire first end face of the main waveguide is connected to a portion of a second end face of the tapered waveguide arranged in opposition to the first end face.
The second condition is that an entire third end face of the first branching waveguide and an entire fourth end face of the second branching waveguide are connected to a portion of a fifth end face of the tapered waveguide, respectively, the fifth end face being arranged in opposition to the third end face and the fourth end face.
With this configuration, the first branching waveguide and the second branching waveguide can be made shorter with respect to the first propagation direction, which corresponds to the input and output directions. Here, xe2x80x9csmooth curved linexe2x80x9d means a curved line that is continuously differentiable. Moreover, by configuring the tapered waveguide as described above, the propagation loss of optical signals propagated along the tapered waveguide can be reduced.
In this embodiment, it is preferable that each shape of the second end face and of the fifth end face is substantially a straight line. The two edges of the second end face are a first edge and a second edge. The two edges of the fifth end face are a third edge and a fourth edge. Under the first condition, the first end face is enclosed by the first edge and the second edge. In the second condition, the third end face and the fourth end face are enclosed by the third edge and the fourth edge.
In this embodiment, it is preferable that, satisfying the first and the second conditions, the main waveguide includes a first boundary line and a second boundary line. The first branching waveguide has a third boundary line and a fourth boundary line, the latter being further away from the second branching waveguide. The second branching waveguide has a fifth boundary line and a sixth boundary line, the latter being further away from the first branching waveguide. At the first end face, the first boundary line is closer to the second edge than the first edge by the distance xe2x80x9cbxe2x80x9d (b greater than 0). At the first end face, the second boundary line is closer to the first edge than the second edge by the distance xe2x80x9cbxe2x80x9d. At the third end face, the fourth boundary line is closer to the fourth edge than the third edge by the distance xe2x80x9caxe2x80x9d (a greater than 0). At the fourth end face, the sixth boundary line is closer to the third edge than the fourth edge by the distance xe2x80x9caxe2x80x9d.
The widths of the main waveguide, the first branching waveguide and the second branching waveguide are for example 8 xcexcm. The thicknesses of the main waveguide, the first branching waveguide and the second branching waveguide are for example 6 xcexcm. The distance between the third boundary line at the third end face and the fifth boundary line at the fourth end face is 3.5 xcexcm. Then the distance xe2x80x9caxe2x80x9d can be 1 xcexcm, and the distance xe2x80x9cbxe2x80x9d can be a constant value in the range of 0 xcexcm less than b less than 1.25 xcexcm.
In the above embodiment, it is preferable that, satisfying the first condition, the main waveguide includes a first boundary line and a second boundary line. The first branching waveguide has a third boundary line and a fourth boundary line, the latter being further away from the second branching waveguide. The second branching waveguide has a fifth boundary line and a sixth boundary line, the latter being further away from the first branching waveguide. At the first end face, the first boundary line is closer to the second edge than the first edge by the distance xe2x80x9cbxe2x80x9d (b greater than 0). At the first end face, the second boundary line is closer to the first edge than the second edge by the distance xe2x80x9cbxe2x80x9d.
In the above embodiment, it is preferable that, satisfying the second condition, the main waveguide includes a first boundary line and a second boundary line. The first branching waveguide has a third boundary line and a fourth boundary line, the latter being further away from the second branching waveguide. The second branching waveguide has a fifth boundary line and a sixth boundary line, the latter being further away from the first branching waveguide. At the third end face, the fourth boundary line is closer to the fourth edge than the third edge by the distance xe2x80x9caxe2x80x9d (a greater than 0). At the fourth end face, the sixth boundary line is closer to the third edge than the fourth edge by the distance xe2x80x9caxe2x80x9d.
The widths of the main waveguide, the first branching waveguide and the second branching waveguide are 8 xcexcm. The thicknesses of the main waveguide, the first branching waveguide and the second branching waveguide are 6 xcexcm. The distance between the third boundary line at the third end face and the fifth boundary line at the fourth end face is 3.5 xcexcm. Then, the distance xe2x80x9cbxe2x80x9d can be 0 xcexcm, and the distance xe2x80x9caxe2x80x9d can be a constant value in the range of 0 xcexcm less than a less than 3.0 xcexcm.
With this configuration, the propagation loss of the optical signal propagated along the tapered waveguide can be reduced.
For the embodiment of the present invention, it is preferable that the curved portion of the first curved line is a circular arc.
In that case, the overall length of the first branching waveguide along the first propagation, which corresponds to the input direction (and also the output direction) is shortened. For the curvature radius of the circular arc, a radius should be selected at which the emission loss is minimal.
For the embodiment of the present invention, it is preferable that the second curved line is a circular arc, and that the curved portion of the third curved line is a circular arc.
In that case, the length of the second branching waveguide with respect to the input and output direction, that is, in the first propagation direction, can be made shorter. For the curvature radii of the circular arcs, radii should be selected at which the emission loss is minimal.
For the embodiment of the present invention, it is preferable that the circular arc of the third curved line is connected to the second curved line.
In that case, the overall length of the second branching waveguide with respect the first propagation direction, which corresponds to the input and output direction, can be made shorter.
For the embodiment of the present invention, it is preferable that the wavelength demultiplexing portion is a reflection-type wavelength demultiplexing portion using a dielectric multilayer filter.
In that case, the grooves for inserting a dielectric multilayer film can be formed by cutting the chips in one straight line on the surface of the series of chips on the wafer on which the wavelength demultiplexers are formed. Consequently, the grooves of the chips for the same series of wavelength demultiplexers can be formed by dicing in one step. Thus, the production efficiency for the wavelength demultiplexer is high.